The right tool might identify problem with fussy Benz

QUESTION: I have a 1985 Mercedes-Benz 380SL with 60,000 miles. It starts fine when cold or if after driving I stop and try to restart in 5 or 10 minutes. But if the car sits longer than about 10 minutes – this could be 2 hours – it will crank and crank and have a difficult time starting. When it does start, it stutters like it is not getting enough gas, or too much. I don't notice any blue smoke from the exhaust, however. If it sits long enough to cool down, it starts fine.

I have been told it is vapor in the fuel line. I have not had a person tell me the problem and solution except to say it could be at least one of three parts and they would have to start replacing expensive parts with a process of elimination. Your help would be appreciated.

–Dick

ANSWER: Whoa! Whatever happened to testing before replacing? Based on your symptoms, I'm leaning towards a fuel injection fault, as this system must properly adapt to engine temperature changes, whereas ignition and mechanical systems don't.

Your Benz utilizes an old-school and almost bulletproof fuel injection system known as continuous injection system, or CIS. It's a mechanical/hydraulic system, differing from more modern electronic fuel injection systems. The key to diagnosing a CIS vehicle is to have a special pressure gauge that allows a look at two different and really important values.

CIS utilizes an air-sensing device, sort of a flapper connected to a hydraulic piston. The more air that enters the engine, the more the piston moves, providing fuel flow to the fuel injectors via the attached fuel distributor – a mechanical/hydraulic marvel. The control pressure regulator, a sometimes buggy device, provides a counter force (a second and opposing pressure) to the top of the piston, altering movement, to allow for cold and warm engine requirements. The pressure gauge can check both overall system and control pressures.

My hunch, based on your symptoms, is possibly the Benz has a control pressure regulator fault or perhaps the cold start injector or one or more of the eight fuel injectors is leaking fuel into the engine after shut off. Both situations could wet the spark plugs, and causing stuttering and/or a no-start. You mentioned no blue smoke was noticed. My scenario could cause some black smoke. Expert use of the fuel pressure gauge would prove or disprove the above possibilities rather quickly, along with checking the fuel pump and fuel accumulator for correct function.

The only other component on this durable system that occasionally causes trouble is the auxiliary air regulator. This gadget adds air during cold starts, providing needed fast idle. When this device ages, some help with the accelerator pedal may be needed during a cold start. Your 1985 version of CIS also includes an oxygen sensor, a management computer and a frequency valve to fine-tune fuel delivery. This part of the system should be checked, but likely isn't the cause of your symptoms.

On the bright side: Should your engine require one of the more expensive components to be replaced, parts are cheap and easy to come by at most self-service auto recyclers. These cars seem to last 10-20 years longer than most, so there's a reasonable number of vintage models mixed in with their newer domestic and Asian counterparts. Functionality isn't assured, but the previously utilized parts can be a very cost-effective gamble.

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ABOUT THE WRITER

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood(at)earthlink.net; he cannot make personal replies.

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